The present invention relates to a method and apparatus for producing milk with a lowered bacterial content, in which raw or whole milk is first divided by centrifugal separation into a fraction consisting of cream and a fraction consisting of skim milk. The skim milk fraction is then subjected to special treatment steps directed to the substantial and efficient reduction of its bacterial content.
Conventional means for producing milk with a lowered bacterial content are known. Swedish patent publication No. 208,841 describes a bactofuge treatment of bacteria-containing milk. The milk is first centrifuged in a bactofuge. The resultant liquid milk fraction enriched in bacteria is sterilized by heating, and then remixed with the other fraction of the centrifuged milk. By this treatment about 90% of the bacteria may be removed and destroyed.
Swedish patent publication No. 380,422 discloses dividing whole milk into permeate and retentate (the term "concentrate" is synonymous with the term "retentate") fractions by microfiltration. The permeate, which is the fraction that passes through the pores of the filter (the size of the pores may range broadly from 0.1.mu.--10.mu.) consists of skim milk (.mu.=micron). The retentate, which is the fraction retained by the surface of the filter, consists of cream. The fat globules and bacteria in the milk are about the same size, and are substantially retained by the filter. Hence, the skim milk obtained is practically free from bacteria. This method of treating milk is effective but such filtration of whole milk is expensive and power demanding. For example, if a microfilter having pore sizes of 0.1.mu. were employed according to the teachings of the '422 reference in order to separate whole milk, the filter pores would become blocked by fat, protein and bacteria very rapidly.
Swedish published patent application No. SE A 67 15081 contains a single claim, which discloses a method for sterilizing milk. According to this claim, the fat is first separated from the skim milk. Next, the fat fraction is sterilized by means of heat, and the skim milk fraction is sterilized by means of bacteria filtering (no filter pore sizes are given). Finally, the sterilized fat and skim milk fractions are remixed to yield a sterile milk product. In order to so sterilize the skim milk fraction by means of bacteria filtering, the pore size in the filter must be so small that no bacteria can pass through it. As can be seen in the Ultrafiltration Handbook, M. Cheryan, p.5 (Technomic Pub. Co., Penna., 1986), the size of the smallest bacteria is less than about 0.5.mu.. Hence, the pores in a filter suitable to achieve the object of the Swedish '081 reference must be even smaller.
Notwithstanding the advantages that would be attained by separately sterilizing fat and skim milk fractions in this manner, the processing costs of such bacteria filtering are very high. Many of the bacteria, fat globules and proteins which are present in the skim milk prior to such filtering are all about the same size. Hence, the pores of a bacteria filter which is effective to so sterilize skim milk will hold back not only the bacteria, but also the fat globules and at least some of the proteins. Such a filter quickly becomes blocked by trapped material. Hence, the flow rate through the filter rapidly declines and the filter must be frequently cleaned or replaced. The high cost of such an inefficient process may be prohibitive.
In some instances it is of particular importance to obtain milk with a lowered bacterial content. For example, a particular batch of raw milk may be so contaminated that mere pasteurization will not result in adequate storage life. For some applications, moreover, it may be of value to be able to provide treated milk in which the bacteria content has been reduced e.g., to about one hundredth of the original value. It is especially important to provide milk with a relatively low bacterial content for the production of cheese, since incorrect bacteria cultures can destroy the cheese. It is normally not suitable to simply heat-treat milk to a sufficient degree for use in cheese production, because such heat treatment may give a lower yield of cheese and can also adversely influence the coagulation time. Conventionally, one usually adds saltpeter in order to avoid incorrect cheese fermentations. Other additions include lysozyme and hydrogen peroxide. In many instances, however, it would be desirable to avoid the addition of saltpeter or other additives.
From the above discussion, it is clear that there is a continuing need for improved milk processing technology which can provide a more nearly sterile product at lower cost.